1. Field of the Invention
The present invention relates to an ink for producing a compound semiconductor thin film to be used for producing, for example, a solar cell, a compound semiconductor thin film produced using the ink, a solar cell having the compound semiconductor thin film, and a process for producing the solar cell.
2. Description of the Related Art
A solar cell is a device which converts light energy into electrical energy using a photovoltaic effect and it has recently attracted attention from viewpoints of global warming prevention and substitutes for depleting resources.
The solar cell is classified roughly into a silicon-based solar cell (monocrystal, polycrystal, amorphous, and their composite), a compound semiconductor-based solar cell (CIS, CZTS, III-V group, and II-VI group compounds), an organic semiconductor-based solar cell, and a dye sensitization-based solar cell according to the kind of the material of a light-absorbing layer which is the most important constituent element. Among them, the CIS (CuInSe) compound semiconductor solar cell is expected as the next generation of solar cell which plays the part of resource saving and energy source for preventing warming because the cell has excellent characteristics such as high optical absorption coefficient of the light-absorbing layer, relatively few production process steps, high radiation hardness, and photoelectric conversion efficiency of more than 19% in a laboratory. Since rare metals are not used for the CZTS (CuZnSnS) compound semiconductor solar cell, it is expected as a low-cost solar cell of the future.
Currently, the light-absorbing layer which is the most important constituent element for the CIS compound semiconductor solar cell and the CZTS compound semiconductor solar cell is formed by, mainly a vacuum process such as vapor deposition or spattering. However, the vacuum process needs expensive vacuum facilities and the production process is also complicated. Thus, the process has a disadvantage of high power costs. There is also a disadvantage that it is difficult to maintain the uniformity of distribution of each element in the surface when forming a film having a large area.
Further reduction in power costs is essential to achieve the spread of the compound semiconductor solar cell. Recently, a method for forming a CIS layer or a CZTS layer by a method for forming a film at low cost such as a printing process has been suggested (see, for example, JP-A 2009-076842 (KOKAI) and Guo et al., J. Am. Chem. Soc., 131 (2009) 11672-11673.). According to the method, the process does not need an expensive vacuum device and the process becomes simple. This may result in a significant reduction in power costs. The distribution of each element in the surface becomes uniform. It is also expected that the conversion efficiency is improved.
However, the method does not use a binder and crystallizes only nanoparticles by an annealing process. As a result, nanoparticles easily aggregate during the coating process, many gaps are present in the crystallized light-absorbing layer, the series resistance becomes high, and the conversion efficiency decreases. Because of no binder, there are problems such that the surface roughness is high, many defects are present on the surface, and the conversion efficiency decreases.